This invention relates generally to calorimetry and in particular to a spectrophotometric method of measuring the heating value of a fuel in the gaseous state. The term heating value as hereafter applied means the amount of heat which would be liberated by combustion from a known quantity of a substance. The term total heating value used herein refers to the aggregate of heating values contributed by each of the constituents of a mixture of substances.
The heating value of a substance is of significant interest because it forms one basis for determining the commercial value of that substance as a fuel. The combustible constituents of a typical gaseous fuel such as natural gas or substitute natural gas (SNG) comprise the hydrocarbons methane, ethane, propane, butane, pentane, and their isomers. Collectively these hydrocarbons, known as the saturated or paraffin hydrocarbons, constitute a homologous series in which members differ by the increment CH.sub.2. In a typical gaseous fuel mixture, methane, CH.sub.4, the simplest constituent, accounts for more than eighty percent (by volume) of the active constituents of the fuel. The heavier paraffin hydrocarbon constituents denoted hereafter as the "ethane-plus" constituents, generally account for the balance of significant combustible components. Before transmission to the distribution mains, typical purification procedures remove undesirable impurities and many of the other hydrocarbons which are present in the unpurified gas. Thus, in determining total heating value, it is often possible to disregard constituents other than the paraffin hydrocarbons.
The conventional methods of measuring heating value known to the art are (1) combustion calorimetry, (2) gas chromatography, and (3) mass spectrometry. Combustion calorimetry is the direct measure of heat liberated by combustion. Gas chromatography and mass spectrometry are techniques of separating and identifying each constituent and measuring the relative concentration thereof. Knowing the heating value of each constituent of a mixture, the total heating value may then be computed. A further method of identifying each constituent and measuring the relative concentration thereof is absorption spectroscopy. However, the absorption spectroscopy method has heretofore found little application to calorimetry because of practical difficulties in accurately measuring the relative concentration of all constituents of interest.
Although all of the aforementioned techniques are of some value in certain applications, each is subject to one or more deficiencies, among which are the following:
1. Operation requiring trained personnel; PA1 2. Delay in obtaining results; PA1 3. Lack of repeatability; PA1 4. Destruction of sample; PA1 5. Cumbersome or expensive instrumentation; PA1 6. Lack of accuracy due to inability to completely distinguish constituents.